1. Field of the Invention
The present invention relates to a transmission system and a transmission/reception system to be used for radio communications, and a local oscillator to be used in the same.
2. Description of the Related Art
The system to be used in the field of radio communications is generally equipped with a reception circuit, in which the frequency of a received signal is lowered into an intermediate frequency signal by frequency converting means and is then demodulated, and a transmission circuit in which the intermediate frequency signal is modulated and then converted into a transmission frequency by frequency converting means and is transmitted.
The two above frequency converting means to be used respectively in the reception circuit and the transmission circuit converts an input signal of a frequency fr into an output signal of a frequency fi by mixing it with a local oscillator output signal of a frequency f1. At this time, the difference between the frequency fr of the input signal and the frequency f1 of the local oscillator output signal is the frequency fi of the output signal. Here, the input signal is the received signal in the reception circuit and the intermediate frequency signal in the transmission circuit. On the other hand, the output signal is the intermediate frequency signal in the reception circuit and the transmitted signal in the transmission circuit.
Now when the received signal of the frequency fr is converted in the reception circuit into the intermediate frequency signal of the frequency fi, a signal component of a frequency fg, located on the opposite side of the frequency spectrum from the received signal with respect to the frequency f1 of the local oscillator output signal, is also converted likewise as the intermediate frequency signal. This signal component is called xe2x80x9cimage frequencyxe2x80x9d, and interferes with the desired signal.
The image frequency fg or the frequency component signal, located on the opposite side from the received signal with respect to f1, will be described in the following. When the frequency fr of the received signal is higher than the frequency f1 of the local oscillator output signal (that is, fr greater than f1), more specifically, the frequency fg of the image frequency is expressed by fg=f1xe2x88x92(frxe2x88x92f1). When the frequency fr of the received signal is lower than the frequency f1 of the local oscillator output signal (that is, fr less than f1), on the contrary, the frequency fg of the image frequency is expressed by fg=f1+(f1xe2x88x92fr)
In order to avoid the interference caused by such image frequency, it is necessary for the system to filter the image frequency component signal out of the received signal at a stage before the received signal is converted into the intermediate frequency signal.
When the frequency f1 of the local oscillator output signal and the frequency fr of the received signal are extremely close to each other, however, a filter having minute passing functionality is required, with such strict design specification, that it becomes difficult to realize. In order to eliminate this disadvantage, there has been known in the prior art the double conversion type system in which the passing functionality requirment of the filter is loosened by performing the frequency conversion two times. The system is advantageous not only in that the filter can be realized relatively easily but also in that the output frequency of the local oscillator and the frequency of the transmitted signal do not come close to each other so that the influences of the transmitted signal upon the local oscillator are reduced, thus stabilizing the characteristics of the local oscillator.
FIG. 8 shows one example of the system which was conceived by us but not realized by us to be already well known in the art. This system is constructed to include a reception circuit 102, a transmission circuit 103, a first local oscillator 130 and a second local oscillator 131.
The reception circuit 102 is equipped with band-pass filters 101 and 106, an amplifier 104 for amplifying a received signal, frequency converters 105 and 107.
The transmission circuit 103 is equipped with a modulator 110, a frequency converter 109 and an amplifier 108 for amplifying a transmitted signal.
The local oscillators 130 and 131 are each constructed of a frequency synthesizer.
The local oscillator 130 is equipped with a voltage-controlled oscillator 111, a frequency divider 112, a variable frequency divider 113, a phase comparator 115 and a loop filter 116. The local oscillator 131 is equipped with a voltage-controlled oscillator 117, a frequency divider 118, a variable frequency divider 119, a phase comparator 121 and a loop filter 122.
The local oscillator 130 stabilizes the frequency by dividing the oscillatory frequency of the oscillator 111 with the dividers 112 and 113, by making comparison with the phase of a reference signal using the phase comparator 115, and by feeding the comparison result to the oscillator 111 through a phase-locked loop [which is composed of the frequency divider 112, the variable frequency divider 113, the phase comparator 115 and the loop filter 116 in that order].
The local oscillator 131 also functions to stabilize the frequency by dividing the oscillatory frequency of the oscillator 117 by the frequency divider 118 and the variable frequency divider 119, by making a comparison with the phase of a reference signal by the phase comparator 121, and by feeding the comparison result to the oscillator 117 through a phase-locked loop [which is composed of the frequency divider 118, the variable frequency divider 119, the phase comparator 121 and the loop filter 122 in that order].
Here will be described the actions of this system.
At time of reseption, the received signal, as received by an antenna 99, is sent to the receiving circuit 102 through a transmit-receive switch 100. This received signal is filtered to remove unnecessary frequency component signals by the band-pass filter 101 of the reception circuit 102 and is amplified by the amplifier 104 and converted into a first intermediate frequency signal by the frequency converter 105. In this case, the frequency converter 105 converts the received signal into the first intermediate frequency signal by mixing it with the output signal of the local oscillator 130.
The first intermediate frequency signal is filtered to remove the frequency component signals (the noise components) other than the desired intermediate frequency component signal by the filter 106 and is frequency-converted into a second intermediate frequency signal by the frequency converter 107. In this case, the frequency converter 107 converts the first intermediate frequency signal into the second intermediate frequency signal by mixing it with the output signal of the local oscillator 131. Finally, the second intermediate frequency signal is outputted to the demodulator (not shown).
At time of transmitting, the output signal of the second local oscillator 131 is inputted to and modulated by the modulator 110 and is then converted into a transmission frequency signal by the frequency converter 109. In this case, the frequency converter 109 converts the output signal of the modulator 110 into the transmission frequency signal by mixing it with the output signal of the local oscillator 130. This transmission frequency signal is amplified by the amplifier 108 and is fed to the antenna 99 through the switch 100.
Such a system is required to have two local oscillators such as the local oscillators 130 and 131, so that its construction is complicated raising the cost.
Here, the prior art that has succeeded in eliminating the defects of the system is exemplified firstly as disclosed in Unexamined Published Japanese Patent Application Nos. 4-87424 and 4-53302.
The first prior art example is equipped, as shown in FIG. 9, with means 141 for multiplying the output signal of a local oscillator 140 so that this single local oscillator 140 is used in a plurality of frequency converters (or mixer circuits) 142 and 143. In FIG. 9: reference numeral 144 designates a variable attenuator; numeral 145 an amplifier; numeral 146 a voltage-controlled filter; numeral 147 an amplifier; numeral 148 a fixed filter; numeral 149 an amplifier; numeral 150 an amplifier; and numeral 151 a controller.
The second prior art example is equipped, as shown in FIG. 10, with means 161 for multiplying the output signal of a local oscillator 160, and means 162 for dividing the frequency, so that the single local oscillator 160 is used in a plurality of frequency converters 163 and 164.
In such prior art examples, unlike the aforementioned double conversion type, the single local oscillator can be used in a plurality of frequency converters so that the construction can be accordingly simplified to lower the cost.
However, the first prior art example has to be newly provided with the frequency dividing means. On the other hand, the second prior art example has to be newly provided with the frequency dividing means and the multiplication means. As a result, these prior art examples are additionally provided with frequency dividing means and multiplication means so that they can avoid neither a increase in the number of parts nor a rise in a cost.
Therefore, a main object of the invention is to provide a reception system and a transmission/reception system which miniaturize the size and lower construction cost by simplifying the construction of means for generating a local oscillation signal.
Another object of the invention is to make it unnecessary to generate a local oscillation signal for another frequency converting means separately by using the output signal of the frequency dividing means in one frequency converting means as the local oscillation signal of the other frequency converting means, thereby simplifying the construction, for miniaturized size and lowered costs.
Still another object of the invention is to make it unnecessary to process the output signal of the frequency dividing means to be the local oscillation signal used in the other frequency converting means by using the output signal of the frequency dividing means of one frequency converting means as it is, as the local oscillation signal of the other frequency converting means, thereby simplifying the construction for miniaturized size and lowered cost.
Still another object of the invention is to make it unnecessary to provide means for eliminating spurious signals by using an output signal of the frequency dividing means known to have little spuriousness, compared with a multiplier or a frequency-multiplying system, as the local oscillatory signal of the other frequency converting means, thereby simplifying the construction, for lowered costs and miniaturized size.
Still another object of the invention is to improve performance, by preventing the close similarity of the frequency of the local oscillation signal of one frequency converting means and the frequency of the transmission carrier wave thereby eliminating the influence of the local oscillation signal upon the transmission carrier wave thus in turn eliminating occurrence of defects such as frequency fluctuations of the transmission carrier wave.
Still another object of the invention is to make it unnecessary to provide means for mixing the local oscillation signal with another signal by using the local oscillation signal of one frequency converting means directly as the transmission carrier wave thereby simplifying construction, for miniaturized size and lowered costs.
Still another object of the invention is to eliminate distortions to improve the characteristics, by amplifying a carrier wave having an unmodulated single frequency and then modulating it at a last stage.
Still another object of the invention is to prevent invasion of noises thereby to improve the characteristics, by closing a second gate of a dual gate FET at receiving time to ensure the disconnection with the transmission side signal line during the receiving action.
In order to achieve the above-specified objectives, according to the invention, there is provided a reception system comprising: first frequency converting means for converting the frequency of a received signal by mixing it with a first local oscillation signal; and second frequency converting means for converting the frequency of an output signal of said first frequency converting means by mixing it with a second local oscillation signal, wherein one of said frequency converting means includes at least: local oscillation signal generating means for generating a local oscillation signal usable by said one of the frequency converting means; and an output unit for generating an internal output signal to be used for the frequency conversion of said one of the frequency converting means, and wherein the other of the said frequency converting means employs said internal output signal outputted from output portion of said one of the frequency converting means, as the local oscillation signal used by said other frequency converting means.
According to the invention, the local oscillation signal to be used in the other frequency converting means is the internal output signal which is also used in frequency conversion by the one frequency converting means, so that the local oscillation signal of the other frequency converting means need not be separately generated, thereby simplifying the construction for miniaturized size and lowered costs.
Further, the internal output signal to be used in frequency conversion by the one frequency converting means is used as it is, as the local oscillation signal of the other frequency converting means, the internal output signal not needing to be processed to be the local oscillation signal of the other frequency converting means, thereby simplifying the construction, for miniaturized size and lowered costs.